1. Field of the Invention
The present invention relates to a fuel cell including an electrolyte electrode assembly and a pair of separators for sandwiching the electrolyte electrode assembly. Each of the electrolyte electrode assemblies includes a pair of electrodes and an electrolyte interposed between the electrodes. The separator has a serpentine reactant gas flow passage on its surface facing the electrolyte electrode assembly. Further, the present invention relates to a fuel cell stack formed by stacking a plurality of the fuel cells.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). Each of the electrodes comprises a catalyst and a porous carbon paper. The membrane electrode assembly is interposed between separators. The membrane electrode assembly and the separators make up a unit of the fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, the fuel gas and the oxygen-containing gas need to be supplied to the entire power generation surfaces of the anode and the cathode between the separators for maintaining the desired power generation performance. Therefore, for example, long reactant gas flow passages (fuel gas flow passage and oxygen-containing gas flow passage) in serpentine patterns are provided on the separators.
In the serpentine reactant gas flow passages, relatively large pressure losses occur for supplying the reactant gases uniformly. Since the reactant gases tend to flow into portions at relatively low pressure, the reactant gases may short-circuit (shortcut) or bypass the intended routes in the serpentine reactant gas flow passages, and the reactant gases may not be supplied to the power generation surfaces sufficiently for the fuel cell reaction.
In an attempt to address the problem, a fuel cell disclosed in the U.S. Pat. No. 6,099,984 has serpentine flow channels in a mirror-image fashion as a passage of the reactant gas. Each of the flow channels is the mirror image of the next adjacent flow channel. Therefore, no pressure difference exists between the adjacent flow channels, and the short-circuit of the reactant gas is prevented.
Specifically, as shown in FIG. 8, a plurality of supply manifolds 2 are formed at an end of a separator 1. The supply manifolds 2 are connected to pairs of first-and second serpentine flow channels 4a, 4b through inlet legs 3a, 3b, respectively. In FIG. 8, one pair of the first and second serpentine flow channels 4a, 4b are shown. The first and second serpentine flow channels 4a, 4b are connected to exhaust manifolds through outlet legs (not shown).
The first serpentine flow channel 4a is symmetrical with the second serpentine flow channel 4b. The first serpentine flow channel 4a allows the reactant gas to flow back and forth in a direction indicated by an arrow H, and flow in a direction indicated by an arrow V1. The second serpentine flow channel 4b allows the reactant gas to flow back and forth in the direction H, and flow in a direction indicated by an arrow V2 which is opposite to the direction V1. Therefore, the reactant gas is supplied to the inlet legs 3a, 3b at substantially the same pressure, and the short-circuit of the reactant gas is prevented.
In the first and second serpentine flow channels 4a, 4b, for example, after the reactant gas flows along a flow groove 5a in the direction H1, and the reactant gas turns back to flow along a flow groove 5b in the opposite direction H2. A pressure difference may exist between the flow groove 5a and the flow groove 5b. Therefore, the reactant gas may short-circuit the flow channel from the flow groove 5a to the flow groove 5b. Stated otherwise, the reactant gas may flow through a gas diffusion layer of the anode or the cathode undesirably. In particular, when the operating pressure is high, or the amount of the reactant gas supplied to the fuel cell is large, the shortcut of the reactant gas frequently happens.
Therefore, in the fuel cell, the reactant gases are not supplied to the entire power generation surfaces uniformly, and the power generation performance is lowered. The mirror image structure of the flow channels is complicated, and the production cost of the separator 1 is high.